The insoluble salt begins to dissolve when you add a third-party agent, giving a pink-colored solution
name: Interaction between potassium permanganate and 18-crown-6 leads to the formation of a complex which is soluble in benzene
Crown ethers are heterocyclic compounds containing more than 11 atoms in their cycles, at least four out of them are heteroatoms linked through an ethylene bridge.
Crown ethers are viscous liquids or crystalline substances, soluble in most organic solvents and poorly soluble in water. Their properties depend on the chemical nature of the functional groups and heteroatoms in the ring.
Crown ethers form stable lipophilic complexes with metal cations. The metal cation is included in an intramolecular cavity of the crown ether and retained by ion-dipole interactions with heteroatoms. Complexes which have the size of the metal ion is close to the size of the cavity of the crown ether molecule are most stable.
Since the diameter of the cavity in a molecule of 18-crown-6 is 2,6-3,2 Å, and the diameter of the potassium ion is 2,66 Å, 18-crown-6 is more specific in relation to this metal.
Inorganic salts are generally poorly soluble in organic solvents. Thus, potassium permanganate is insoluble in benzene, but its complex with the crown ether is soluble. This feature of the complex allows to perform chemical reactions in organic media using potassium permanganate.